Introduction

In Mexico, fresh corn consumption is part of the multiple uses of maize in the Mexican diet, as it has a culinary tradition and many popular festivities, it can be consumed roasted, boiled, in soups, puddings, or canned. This tradition has been inherited from Mesoamerican people and pre-Hispanic cultures to Mexicans for hundreds of years. The Mexican natives were the ones who made maize evolve, formed the native breeds, and through interracial crosses, gave rise to modern landraces, from which the current high-yielding hybrids have been obtained (^{Márquez-Sánchez, 2008}). In this sense, the native races of maize that stand out as fresh corn in western Mexico are Jala, Tabloncillo, Bofo, Harinoso de Ocho, Elotero de Sinaloa, Elotes Occidentales, and Dulce (^{Ortega-Paczka, 2003}; ^{Ron-Parra et al., 2006}), which were empirically selected by Mexican farmers for this purpose.

Fresh corn ears production represents advantages over grain production; the production cycle is shortened, ear rot and storage pests are avoided, and the rest of the plant can be used for forage (^{Ortiz-Torres et al., 2013}). In 2020, an area of 69,893 hectares of corn was cultivated in Mexico for fresh corn production with a yield of 947,998 tons and an average production of 14.6 t ha^-1; the states of Puebla (15,599 ha), Jalisco (8,105 ha), and State of Mexico (4,907 ha) were the main producers (^{SIAP, 2020}).

Currently, in the country there is a lack of improved maize especially for the fresh corn ear market, although farmers use native seeds to supply local markets, as is the case of varieties of Cacahuacintle (^{Jasso-Bobadilla et al., 2019}; ^{Osorio-Saenz et al., 2019}), Chalqueños and Elotes Cónicos in central Mexico (Osorio-Saenz et al., 2019) or plant commercial maize hybrids with suitable attributes for commercialization, such as the case of Monsanto's A-7573 for larger areas (^{Andrés-Meza et al., 2017}).

In general, maize breeders have made limited use of the diversity safeguarded by Mexican farmers, since they work mainly with four landraces, Chalqueño and Cónico for the Mesa Central, Celaya for the Bajío, and Tuxpeño for the Gulf region (^{Márquez-Sánchez, 2008}); however, there are 49 racial groups in Mexico (^{Sánchez & Goodman, 1992}), with a wide possibility of use to make genetic improvement with a specific quality. In this sense, ^{Coutiño-Estrada et al. (2010a}) identified in the state of Chiapas maize of the Tuxpeño race with excellent traits for fresh corn, especially for length and sweet flavor due to its sugar content (up to 13.8 ° Brix). Meanwhile, ^{Ortiz-Torres et al. (2013}) reported high fresh corn yield variation and other quality indicators among maize populations in the Tehuacán Valley, Puebla.

By utilizing the diversity of native Mexican maize, fresh corn quality could be improved through an intrapopulation recurrent selection scheme, which increases the frequency of genes for quantitative traits that accumulate in selection cycles (^{Hallauer et al., 2010}), as proposed by ^{Coutiño-Estrada et al. (2010b}) in native populations of the Tuxpeño race and ^{Valdivia-Bernal et al. (2010}) in a maize of the Jala landrace, where they found that additive effects were the most important for sweetness and fresh corn length in a diallel cross. However, fresh corn ear quality traits are also caused by the wide variability of pericarp thickness and endosperm starch texture (^{Paliwal et al., 2001}), so they should also be considered as selection criteria in a breeding program.

^{Márquez-Sánchez (1990}) proposed the limited backcross breeding method to use and increase grain yield in native populations of maize, and consists of crossing improved varieties (donor) with native populations (recurrent) and subsequently making only one backcross to the original population. ^{Márquez-Sánchez et al. (2000)} called these original populations with backcrossing, – improved varieties and pointed out that there is a grain yield superiority of up to 25.6 % over their original counterparts. With this methodology, Vázquez-Sánchez et al. (2003) developed improved maize varieties with higher yields than the original native variety, up to 30 %, and meet the quality specifications of the native varieties.

Currently, there is limited information on fresh corn ear varieties formed from any breeding scheme using native maize. The limited backcrossing scheme proposed by ^{Márquez-Sánchez (1990}) and applied by ^{Márquez-Sánchez et al. (2000)} and ^{Vázquez-Carrillo et al. (2003}), among others, has focused on increasing grain yield with the direct use of native maize, without pre-breeding. Therefore, the objective of the present work was to evaluate the potential and quality of fresh corn ear from maize populations formed from the best half-sib families of a maize composite of the Jala race that were backcrossed to an improved variety.

Material and Methods

Results and Discussion

In the combined ANOVA, significance (p ≤ 0.01) was detected among genotypes (Table 1), for seven of the variables analyzed: silking and tassel days, plant height, ear height, fresh corn length, Brix degrees, and fresh corn ear yield. The fresh corn diameter, peduncle length and percent moisture content were not significant. The detection of statistical differences among genotypes indicates the presence of variability among them, as reported by formerly reported by other authors (^{Coutiño-Estrada et al., 2010a}, ^{Martín-López et al., 2008}).

Between locations (Table 1) there were significances (p ≤ 0.01) for the variables of silking days, tassel days, plant height, ear height, fresh corn ear length, peduncle length, Brix degrees, and fresh corn yield. Fresh corn diameter and percent moisture content were not significantly different. The significance of the variables among locations corroborates the influence of the environment on genotypes in the overall average fresh corn quality and yield, as observed by ^{Ortiz-Torres et al. (2013}).

The Gen x Loc interaction was significant (p ≤ 0.01) in four variables: silking and tassel days, plant height, and fresh corn yield. These differences in the behavior of genotypes across environments should be taken into account to avoid discarding varieties that may be superior in a given environment, as observed by ^{López-Guzmán et al. (2018}) in Jala maize populations. However, this research was conflicting with reports in a similar study performed by ^{Coutiño-Estrada et al. (2010b}) and ^{Martín-López et al. (2008}) for most of the fresh corn quality variables.

Regarding silking and tassel days, the variety C₁SMJ was statistically (p ≤ 0.05) superior to other genotypes (Figures 1A and B), the backcrosses to the parent (C₁SMJ) had an SD between 73 and 76 days and a TD between 75-78 days, while the group of varieties with – of TX UAN presented between 71 to 74 days for SD and between 72 to 76 days for TD (Figure 1A and B). These results indicate that the performance shown by these genotypes was intermediate when compared to its parents. The commercial hybrids A 7573 and H-561 showed an early performance, 65 and 67 days, respectively. By environment, it was observed that the varieties were earlier in San José de Mojarras than in the other locations, possibly due to the delayed sowing date and the lower altitude of this location (Figures 2A and B).

Backcrosses to the C₁SMJ (donor) parent families were 32 and 55 % higher in plant and ear than the commercial controls (A-7573 and H-561) but 21 and 27 % lower in plant and ear than the C₁SMJ variety, which was statistically the highest genotype (Figures 1C and D). On the other hand, the backcrosses to the recurrent donor (TX UAN) were 25 and 61 % taller in plant and ear than the commercial controls that presented on average 227 and 97 cm, respectively. These varieties were very similar in plant height to the TX UAN parent; however, 56 % of these backcrosses had a lower ear height (Figures 1C and D), so the genetic improvement achieved by limited backcrossing using half-sib families selected from the C₁SMJ variety was effective to obtain varieties with better agronomic traits.

Means with the same letter are statically equal (Tukey, 0.05).

Figure 1 Silking days (A), Tassel days (B), Plant height (C), Ear height (D), and Fresh corn ear length (E) in maize population selected from a composite of race Jala. 

Means with the same letter are statically equal (Tukey, 0.05).

Figure 2 Silking days (A), Tassel days (B), Plant and ear height (C), Fresh corn ear, and peduncle length (D) in environments. 

The environment where plant and ear height were best expressed was San José de Mojarras, with an average of 308 and 161 cm, respectively (Figures 1C and 1D); while in the locality of Jala, plants were smaller, with an HP of 247 cm and HE of 120 cm, possibly attributed to low humidity and fertility of the soils (^{Aguilar-Castillo & Carballo-Carballo, 2007}).

In the fresh corn length variable, the C₁SMJ parent was the largest with an average of 25.2 cm, but statistically equal to the varieties RC-302, RC-307, and RC-301. Meanwhile, 25% of the backcrosses statistically outperformed the control (A-7573) which showed the smallest fresh corn at 18 cm long (Figure 1E). These results indicate that the selection in families made in variety C₁SMJ made it possible to better control the ear length trait; hence, it is important to perform pre-breeding in native varieties and subsequently cross them with improved maize, as proposed by ^{Coutiño-Estrada et al. (2010a}). In addition, it is important to highlight the importance of the Jala landrace as a source of alleles to obtain varieties with greater fresh corn length, as suggested by ^{Valdivia-Bernal et al. (2010}). In general, the backcrosses evaluated showed adequate sizes for the fresh corn market, since according to ^{Tracy (2000}) the preferred type size is about 20 to 23 cm. With respect to the environments, the largest sizes were obtained in Xalisco (23 cm) and in Tetitlán, with the lowest average (19 cm); the above was a reflection of the behavior of the varieties in the evaluation environments, as observed by Coutiño-Estrada et al. (2015) in native varieties from Chiapas.

In soluble solids content, 69 % of the backcrosses were statistically equal to the commercial controls A-7573 and H-561, which presented the highest averages with 10.0 and 9.96 ° Brix (Figure 3A). The backcrosses with 25 % germplasm of the Jala variety obtained on average higher soluble solids content (8.21 °Brix) than those formed with 75 % of the C₁SMJ variety (7.6 °Brix). The results were like those of the parents, TX UAN and C1SMJ, but very different from the values reported by ^{Valdivia-Bernal et al. (2010}) for a local population of the Jala landrace (2.53 °Brix) or for the hybrid A-7573 (1.65 °Brix), possibly due to the equipment used to measure soluble solids in their study. Between environments, it was observed that the highest Brix content was obtained in the Tetitlán locality, while the lowest values were observed in Jala and Xalisco, a consequence of the variability between localities (Figure 4A), as reported by ^{Coutiño-Estrada et al. (2015}), in similar work.

Observed differences in the grain soluble solids content could also be caused by the grain maturity stage at the moment that the measurement was made. In this sense, the commercial corn hybrids were analyzed at the R4 maturity stage, while the backcrosses and their parents at the R3 maturity stage. These details should be considered when measuring total soluble solids since ^{Hale et al. (2005}) mention that the maturity stage of fresh corn affects sucrose and total sugars in sweet corn with the su, se, and sh ₂ genes since they have higher values at later stages and the concentration varies between and within phenotypes. Therefore, it can be inferred that the criteria for quantitative quality of fresh corn ear in Mexican maize vary between and within populations, as well as between types of corn, and to date, what prevails in the country's supply markets is mainly limited to a uniform size of fresh corn and that it has a longer shelf life.

Means with the same letter are statically equal (Tukey, 0.05)

Figure 3 Grades Brix (A) and Fresh Corn Yield (B) in maize population selected from a composite of race Jala. 

In fresh corn ear yield, from the 16 RC₁F₂ backcrosses evaluated, six formed the superior statistical group and when comparing the backcrosses with the commercial control A-7573, 58 % exceeded it on average with 2.156 t ha^-1 (Figure 3B). This result evidences the genetic contribution of family selection in the Jala variety (C₁SMJ) to form the backcrosses that were the basis of this study, where RC205 stood out with 16.541 t ha^-1 (Figure 3B) and supports that the previous improvement made in the germplasm of the Jala race was adequate to form corn populations. Regarding locations, San José de Mojarras showed the highest yield with 16.9 t ha^-1, which confirms the influence of the environment on fresh corn yield, as observed by ^{Ortiz-Torres et al. (2013}), in previous evaluations.

Means with the same letter are statically equal (Tukey, 0.05).

Figure 4 Grades Brix (A), Moisture (B), and Fresh corn Yield (C) in environments. 

Conclusions

The improved populations were generally later between 6 and 11 days than the commercial control fresh corn (A-7573) and were taller in plant and ear height. Twenty-five percent exceeded the A-7573 control in fresh corn length and 69% of the varieties were statistically equal in soluble solids (10 °Brix). For fresh corn ear yield, 58 % of the backcrosses surpassed the control A-7573 with 2.156 t ha^-1, which showed that the RC₁F₂ evaluated from the family selection practiced in the Jala variety (C₁SMJ) promoted the accumulation of favorable alleles for fresh corn size and quality, hence it is suggested that to achieve better results, it is necessary to make pre-improvement in native varieties and subsequently cross them with improved maize to obtain populations with better agronomic traits and higher fresh corn yield.